Origin of NO
NOx is a generic term for the various nitrogen oxides produced during combustion. Nitrogen oxides are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant, and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems. Consequently, the sources of NOx emissions are now being subjected to more stringent standards. In atmospheric chemistry the term NOx means the total concentration of NO, NO2, N2O, N2O3 and N2O5.
Nitrogen oxides can be formed during the combustion of nitrogen precursors in the fuel, defined as fuel NOx, but also from the nitrogen in the air via two mechanisms, one designated as thermal NOx, via the Zeldovich mechanism:O+N2→NO+N  (1.1)N+O2→NO+O  (1.2)N+OH→NO+H  (1.3)
The other is designated as prompt NOx, where the nitrogen in air is fixed by hydrocarbon radicals and subsequently oxidized to NOx [G. Löffler et al. Fuel, vol. 85, pp. 513-523, 2006]:N2+CH→HCN+N  (1.4)
Three primary sources of NOx formation in combustion processes are documented: Prompt NOx, fuel NOx and thermal NOx [C. S. Latta Plant Engineering, vol. 52 (10), pp. 105-110, 1998]. Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Due to the break-up of the nitrogen triple bond (i.e. reaction (1.1)), thermal NOx is primarily produced at high temperatures, usually above 1200° C. [H. Bosch et al. Catal. Today, vol. 46, pp. 233-532, 1988].
From a thermodynamic point of view, the reaction N2+O2→2NO is very unfavoured with an enthalpy of ΔH° 298 K=−452 kJ [G. Busca et al. Catal. Today, vol. 107-108, pp. 139-148, 2005]. Therefore it requires very high temperatures to proceed at a reasonable rate. From a pure chemical equilibrium observation, it is obvious that the formation of the various nitrogen compounds, N2, N2O, NO or NO2, is proportional with the oxygen partial pressure, due to the increasing O/N ratio.
Another source of NOx production from nitrogen containing fuels, such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. The nitrogen bound in the fuel is here released as a free radical and ultimately forms free N2 or NO, through the following reaction [G. Busca et al.]:4NH3+5O2→4NO+6H2O  (1.5)where the nitrogen containing compounds, like ammonia and amines, are oxidized to NO. The reaction is thermodynamically highly favoured, with an enthalpy at ΔH° 298 K=−452 kJ; although less favoured than the oxidation to N2. The amount of formed ‘fuel NOx’ primarily depends on the amount of nitrogen in the fuel, and also strongly influenced by the reactor design. In natural gas (methane), nitrogen compounds are virtually absent, but substantial amounts of nitrogen is present in the case of coal, gas oils and fuel oils as well as biofuels, such as wood [Busca].
Prompt NOx is generated when the fuel-to-air ratio is high where nitrogen radicals formed in reaction (1.4) react with oxygen via reaction (1.2). The reactions are almost non-temperature dependent, but the prompt NOx formed is negligible relative to thermal NOx.
Methods of Nitrogen Oxides Removal
The numerous possibilities to reduce NOx can be divided into three categories: Precombustion, combustion modifications and post combustion [Latta]. The precombustion strategy imply using alternative fuels with a lower content of nitrogen species [Busca]. During combustion different types of modifications can be utilized, of which the most used are: Low NOx-burners, reburning and staged air combustion (thermal oxidation) [Latta]. A variety of other methods is also possible in the combustion modification: Burners out-of-service, derating, burner system modification, trim and diluent injection; all described by Latta. Several post-combustion approaches are applied to reduce NOx: SCR, selective noncatalytic reduction (SNCR), adsorption, NOx recycle, direct decomposition [Latta], photocatalytic oxidation [J. Dalton, et al Environmental Pollution, vol. 120, pp. 415-422, 2002], multifunctional filter (removal of fly-ash and NOx) [D. Fino et al. Chem. Eng. Sci., vol. 59, pp. 5329-5336, 2004] and pulse intense electron beam irradiation.
One of the most widespread technologies for removing NOx from flue gases is the selective catalytic reduction (SCR) process employed in stationary sources or power plants due to its efficiency and economy. The SCR process for removing nitrogen oxides is based on the reaction between NOx and ammonia:4NO+4NH3+O2→4N2+6H2O  (1.6)NO+NO2+2NH3→2N2+3H2O  (1.7)
In a typical application, ammonia is injected into the NOx-containing gas and the mixture is passed through a flow distribution system and one or several catalyst layers. The main components of an SCR DeNOx system include a reactor with catalyst and an ammonia storage and injection system.
Many different supports and catalytic metals are utilized for the SCR process, but the vanadia/titania catalyst is traditionally applied because of its thermal stability and resistance towards sulfur poisoning [N. Topsoe et al J. Catal., vol. 151, pp. 226-240, 1995.].
The ammonia source can be either anhydrous ammonia, ammonia water or a solution of urea. Because of its better performance ammonia is often utilized, but due its poisonous character and difficult handling, urea can be used, although not quite as effectively as ammonia.
The ammonia is evaporated and subsequently diluted with air or a flue gas side stream before it is injected into the flue gas duct upstream the SCR reactor. Direct injection of ammonia water or a urea solution is also possible. The SCR process requires precise control of the ammonia injection rate and a homogeneous mixing into the flue gas to ensure efficient NOx conversion without an undesirable release of unconverted ammonia referred to as ammonia slip. The SCR process typically requires a temperature of about 350-400° C.
Urea is often used in mobile units, where e.g. the ammonia slip would be avoided. Besides urea as an alternative to ammonia in the SCR process, it is also possible to use hydrocarbons. The possibility for reducing NO with hydrocarbons such as olefins and higher alkanes was first proposed in 1990 [Busca]. Hydrocarbon-SCR systems use hydrocarbons as the reductant. The hydrocarbon may be present in the exhaust gas or it may be added to the exhaust gas. This has the advantage that no additional reductant source (e.g. urea) needs to be carried on-board, but these systems cannot offer the performance of ammonia-SCR systems. In stationary plants, methane is the preferred choice for NOx removal from flue gases from power stations because it is already present, at least in methane (natural gas)-fueled plants.
Although the catalytic removal of NOx (nitrogen oxides, covering NO, N2O, NO2) from the flue gas is a very effective process, the overall high operating expenses of the SCR process and possibility of ammonia slip have motivated a search for other methods to abate emissions of nitrogen oxides.
A different concept is presented by wet scrubbing systems for removal of SO2 and NOx. Some aqueous scrubbing systems have been developed for the simultaneous removal of NOx and SO2 [C.-L. Yang et al. Environmental Progress, 17, 80-85 (1998)].
The wet flue gas desulfurization (FGD) typically exhibits high SO2 removal efficiencies, but the FGD can only remove a small amount of NOx because about 90-95% in a typical flue gas is present as insoluble NO and only the remaining 5-10% NO2 is water soluble. Attempts to oxidize NO to water soluble NO2 have been made by adding strong oxidizing additives, such as MnO4− and H2O2, but the treatment cost involved herein has been too high for practical utilization.
Promising results of the simultaneous NO and SO2 removal in a [Co(NH3)6]2+ solution, which operates below 80° C., have been reported by Long et al. [X.-I. Long et al., Industrial & Engineering Chemistry Research, 43, 4048-4053 (2004)].
Another approach for removing NO is the complexation of NO with Fe2+-chelates based on ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetate (NTA), as outlined in reaction 1.8 for the case of iron-EDTA complex [F. Ron-caroli et al., Coordination Chemistry Reviews, 251, 1903-1930 (2007)].FeII(EDTA)+NO⇄FeII(EDTA)(NO)  (1.8)
The metal-chelate can be electrochemically regenerated after absorption or reduced by sulfite ions to sulfate and nitrogen [F. Gambardella et al., Industrial & Engineering Chemistry Research 44, 4234-4242 (2005)].
In U.S. Pat. No. 6,235,248 a biotechnological approach to regenerate the iron-complex, the so-called BioDeNOx process was described. In this process the NO-saturated iron-chelate solution is brought in close contact with bacteria that regenerate the iron-EDTA complex and convert the bound nitrosyl to N2. The FeII(EDTA) solution needs to be somewhat diluted (concentration <200 mM) due to the presence of microorganisms, which naturally limits the absorption capacity.
The above proposed technologies for NO removal are all associated with various challenges such as: low capacity, large installation footprint, poor reaction kinetics, hazardous stoichiometric reductants or oxidants, elevated reaction temperatures and the requirement for specialized catalysts.
Many of the above proposed technologies are further based on liquids with a vapour pressure, which means that the solvent to some extent vaporizes during operation. One promising solution to this particular problem could be the use of a relatively new class of solvents referred to as ionic liquids (ILs). The expression ‘ionic liquid’ in principle encompasses any liquid entirely composed of ions (e.g. molten salts). However, within the context of this work the term will only be used to describe materials which are liquid in their pure state at room temperature. This class of solvents is often considered as ‘green’ solvents because of their immeasurably low vapour pressure. This feature gives the ILs an essential advantage over traditional solvents used for absorbing gases. Ionic liquids have already demonstrated promising behaviour in a number of reactions where gaseous reactants enter the IL solution (such as hydrogenation, hydroformylation, and oxidations) despite low gas solubilities of the gases in the IL at ambient conditions [J. L. Anthony et al. The Journal of Physical Chemistry B, 106. 7315-7320 (2002)].
Another known application of ILs is to utilize them to separate gas mixtures. Patent application WO 2007/101397 comprises gas purification processes and mentions a broad range of ionic liquids as possible absorbers of many different gasses, but does not provide any experimental evidence supporting these propositions. WO 2007/101397 is merely a theoretical review since there is no data evidencing how the ionic liquids work. Recently, a promising solid ionic cation (1,1,3,3-tetramethylguanidinium) has been identified for the absorption of SO2 [J. Huang et al., Journal of Molecular Catalysis A: Chemical, 279, 170-176 (2008)]. Anthony et al. [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002)] reported of the solubilities of a number of gases (such as CO2, CO, O2) in imidazolium-based ILs.
Ionic liquids tend to be more viscous compared to conventional solvents, however, which can result in challenges regarding the mass transfer of gas into the IL. In the case of low-soluble gases, the mass transfer into the IL will likely be a rate limiting step, which can be minimized by increasing the interfacial gas-IL area and/or use high pressure systems [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002)].
So far, only limited information regarding the gas solubilities in ILs has been reported. Besides the reports regarding CO2 capture, the focus of most work revolves around the reactions taking place in the IL with the gas already absorbed. Only few reports exist on gas solubilities [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002); J. L. Anderson et al., Accounts of Chemical Research, 40, 1208-1216 (2007)]. The Brennecke group has, e.g. contributed with a number of seminal studies on absorption of a number of gases in imidazolium-based ILs [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002); J. L. Anderson et al., Accounts of Chemical Research, 40, 1208-1216 (2007); J. L. Anthony et al., The Journal of Physical Chemistry B, 105, 10942-10949 (2001); J. L. Anthony et al., The Journal of Physical Chemistry B, 109, 6366-6374 (2005)].
Consequently, there is still a need for developing efficient processes for removing NOx, and specifically the most abundant NOx component NO, from flue gasses from not only large stationary sources like power or incineration plants, but also from mobile emission sources like, e.g. commercial marine vessels which require a small installation footprint, low energy consumption and preferably no carrying of hazardous chemicals.